Microencapsulation is a well known method used to formulate water-immiscible materials. Microcapsules are usually designed to prevent their active ingredients from permeating the capsule wall during the storage period before use. For example, the active ingredient present in microcapsules which are formulated as a suspension concentrate may leak from the capsules during storage and cause undesirable changes in the toxicological properties and/or physical properties of the suspension. In particular, active ingredient leakage may effect the flow, viscosity and appearance of the suspension, and lead to crystal growth. Crystal growth is especially problematic because it increases the viscosity of the suspension, causes sediment formation in the suspension and may cause blockages in the equipment used to distribute the suspension.
In the past, thick walled microcapsules have been used to prevent permeation of water-immiscible active ingredients. However, thick walled microcapsules are not entirely satisfactory because they do not readily and/or completely release their active ingredients when used for their intended purpose.
Certain pH-sensitive poly(styrene) microcapsules are described by Kokufuta, et al in Biotechnology and Bioengineering, 32, 289-294 (1988). That paper discloses that poly(styrene) microcapsules are coated with a polyelectrolyte to make them pH sensitive. However, water-immiscible materials cannot be encapsulated by the process described by Kokufuta, et al.
Poly(L-lysine-alt-terephthalic acid) microcapsules which exhibit pH dependence are described by Miyauchi, et al, in Journal of Microencapsulation, 9 (3), 329-333 (1992). However, that paper only describes the preparation of water-containing microcapsules and does not disclose a method for the preparation of microcapsules containing a water-immiscible material.
It is, therefore, an object of the present invention to provide microcapsules which contain water-immiscible materials and rapidly release their contents when used for their intended purpose.
It is also an object of the present invention to provide storage stable, aqueous microcapsule compositions which do not readily crystallize.
Those and other objects of the present invention will become more apparent from the detailed description thereof set forth below.